Neurotoxin production and ichthyotoxicity of Chattonella antiqua isolated from Yatsushiro Sea, Japan in 1978 were investigated at different temperatures and light intensities under laboratory conditions. Variation in temperature had a pronounced effect on toxin profiles of the flagellate. The yields of CaTx-II (corresponding to PbTx-2) and CaTx-III (corresponding to PbTx-3) peaked at 15 C with 0.39 and 3.18 pg/cell. As the temperature increased, the amount of CaTx-II and CaTx-III decreased gradually, but there was an increase in the amount of CaTx-IV (corresponding to oxidized PbTx-2). The concentration of CaTx-I (corresponding to PbTx-1) was high at 20 C (0.94 pg/cell). The sharp decrease in all fractions were found at temperatures above 25 C where the organism showed a little growth. Light intensity had a smaller influence on toxin profiles of this species. The yields of CaTx- I and III at 20 mE m-2 sec-1 were 0.86 and 1.84 pg/cell and decreased slightly with increased light intensity. The concentration of CaTx-II remained nearly constant between 20 and 100 mE m-2 sec-1 and lowered at high light intensities. Ichthyotoxicity showed the highest toxicity at 15 C and markedly reduced as the temperature exceeded 20 C. Cells cultured at 25 C were about 2 times less toxic than those cultured at 15 C.